Vacuum tube amplifier



Nov., 938 's BALLANT-[NE 2,134,747

VACUUM TUBE AMPLIFIER IN VEN TOR.

BY @y m;

` ATTORNEYS.

Nov. l, 1938.

VACUUM TUBE AMPLIFIER S. BALLANTINE 5 Sheets-Sheet. 2

Filed March :51. 1900 We W@ lIN V EN TOR.'

s. BALLANTINE VACUUM TUBE AMPLIFIER Nov. l, 1938.

Filed March 51. 1930 IN V EN TOR.

@fila/z MM,

A TTORNEYS.

CII

Patented Nov. 1, 1938 UNITED STATES PATENT OFFICE VACUUM TUBE AMPLIFIERtion of Delaware Application March 31,

21 Claims.

'I'his invention relates to vacuum tube ampliers and particularly toradio receivers employing radio frequency ampliiiers.

An object of the invention is to provide a vacuum tube amplifierincluding an improved system for controlling the output level. A furtherobject is to provide a vacuum tube amplifier including volume controlapparatus effective to prevent distortion at high signal inputs and topreserve a maximum signal to noise ratio. More specifically, an objectis to provide a vacuum tube amplifier in which the control of volume isefiected by changing simultaneously the amplilication of the vacuum tubeamplifier and the magnitude of the signal voltage impressed upon theamplifier.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims, the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin `connection with the drawings in which I have indicateddiagrammatically one circuit organization whereby my invention may becarried into effect. i,

These and other objects will be apparentY from the followingspecification when taken with the accompanyingy drawings in which:

Figs. 1 and 2 are curve sheets showing the variation of shot-noise withamplification, and

Fig. 3 is a circuit diagram of a radio receiver embodying the invention.

In conventional radio receivers, the control of volume or output levelhas been effected in one of two ways, i. e., the input voltage has beencontrolled by a voltage divider which was adjusted to impress anydesired portion of the antenna voltage upon the first amplifier tube, orthe full Aantenna voltage has been impressed upon the amplifier andtheradio frequency amplification has been controlled by regulating thedirect current potential applied to a tube element, for eX- ample, tothe control grid. Both of these methods possess certain disadvantages.

In the rst vacuum tube in the radio fre'- quency amplifier there arenatural statistical fluctuations in the electron How between cathode andplate which, as pointed out by W. Schottky (Annalen der Physik, vol. 57, 541, 1918), cause characteristic oscillations in the tunedv circuitsassociated with the tube. In practice these oscillations are ordinarilyof such small amplitude as to be inaudible in the absence of an incomingsignal, but when a carrier or signal is present,

1930, Serial No. 440,514

they beat with this carrier and produce a noise in the loud-speaker ortelephones. With a square-law detector, this noise is proportional tothe carrier, or signal. The noise resembles the sound of escaping steamand is called shot- 5 effect noise or briey shot-noise. Other causes ofnoises in tubes and circuits also exist, but for convenience they willall be referred to here as shot-noise.

Since the shot-noise is dependent upon the carrier voltage, it isevident that it will be constant regardless of the magnitude of theincoming signal wave so long as 'the input to the rst amplifier isadjusted for a constant signal voltage (or constant carrier voltage) atthe detector. The result of this is a constant signal to noise ratio inthe reception of all stations whether local or distant when the volumeis adjusted by means of antenna coupling. This constitutes a decideddrawback of the method as will be seen presently. i

Now turning to the other method, I have discovered experimentally thatin general any adjustment of electrode voltages in a vacuum tube whichreduces the amplification also reduces the noise in proportion.Experimental evidence of this in the case of an amplifier employingtetrodes is shown in the curves reproduced in Figs. 1 and 2. Fig. 1shows this close relation between shotnoise and amplification when thecontrol grid voltage is varied.

Curve A shows the relationship between the amplication and the controlgrid bias; the potentials applied to the other electrodesbeing constant.Curve B shows the relationship between shotnoise and control grid biasfor the same amplifier. In Fig. 2, the curves C and D represent thevariation of amplification and of shot-noise, respectively, with platevoltage. In all cases, the ratio of amplification to shot-noise remainsapproximately constant over the useful part of the control range. Thesignicance of this is that in controlling for constant output, thesignal remains constant while the shot-noise decreases continuously asthe strength of the incoming Wave increases, thus resulting in animprovement of the signal to noise ratio for local stations. It will beclear that from the viewpoint of noise production this method issuperior to the othe the superiority increasing with the incoming signalstrength. The method has, however, the disadvantage that as the voltageon the input of the amplier tubes increases, a point is soon reached atwhich the relation between input and output voltages becomes non-linearand distortion results.

I propose to retain the advantage of high signal noise ratio of theamplification control method, and to avoid the distortions which occurin this method when the signal voltage is large by employing acombination of the amplification control and input voltage controlmethods. This may be done by employing two controls, one of each type,the two controls being so related that, with increasing signalstrengths, the antenna control prevents the input to the first amplifiertube from rising above the Value which would produce distortion in theamplifier. It is preferable to simplify the receiver operation bylinking these controls in such a way as to require but one knob foradjustment.

Referring to Fig. 3 which illustrates an application of the invention toa typical radio receiver, the reference numeral l identifies the voltagedivider which determines the proportion of the antenna voltage which isapplied to the first amplifier tube. The amplification of the first tubeis controlled by the control grid bias which is determined by theVoltage drop acrosS that portion of the voltage divider 2 which isincluded between the contact arm 3 and the grounded side of the voltagedivider 2. This contact arm 3 and the contact arm 4 of the antennacontrol are mounted upon a common operating shaft 5 which carries a knob6 or other convenient adjusting means.

S0 far as concerns the present invention, the detailed construction ofthe radio receiver is not important and may follow any usual Vor desireddesign. The control of amplification may be confined to a single stage,as in the circuit shown in Fig. 3, or may include two or more stages. Itis also to be understood that the amplification control may be obtainedby varying the screen grid or the plate potential since, as shown byFigs. l and 2, a decrease in ampliflcation reduces the shot-noiseregardless of the particular op-' erating potential which is adjusted tocontrol the amplification.

It will be apparent that there is considerable latitude in theconstruction of amplifiers embodying the invention.

While I have indicated and described one arrangement for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationshown and described, but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appendedclaims.

I claim:

1. The method of controlling the output level of a vacuum tube amplifierand increasing the signal to noise ratio which comprises simultaneouslyadjusting the amplification of the amplifier and the magnitude of thesignal voltage impressed upon the same.

2. In the operation of a radio Vreceiver having a radio frequencyamplifier, the method of increasing the range of signal wave voltagesthat may be transmitted without distortion, which comprisessimultaneously reducing the proportion of the received signal voltagewhich is impressed upon the amplifier and the amplification of saidamplifier.

3. In a carrier wave amplifier, the combination with means for adjustingthe amplification of said amplifier, of means for adjusting theproportion of received signal voltage to amplifier input voltage, andmeans for simultaneously adjusting the said input and amplicationcontrol means.

4. In a radio receiver, a voltage divider for determining the ratio ofantenna voltage to amplifier input voltage, a voltage divider fordetermining the potential applied to a volume coritrol element of theamplifier, and means linking the movable contacts of said voltagedividers to insure the simultaneous adjustment thereof.

5. In a radio receiver, a radio frequency amplifier, an antenna controlfor regulating the input from a collector structure to said amplifier, acontrol for adjusting an operating potential applied to said amplifierto control thereby the amplification of said amplifier, and meanslinking said controls to reduce the input to said amplifier as theamplification thereof is reducedn 6. The combination with an electricalsignal amplifier having an input circuit, an output circuit, and atleast one electric discharge amplifier device connected between saidcircuits for the transmission of signals from said input to said outputcircuit, of means for varying the amplification of the amplifier device,means for varying the signal strength applied to said input circuit, andadditional means for varying said last named means simultaneously withsaid first named means.

7. The combination with an electrical signal amplifier having an inputcircuit, an Output circuit, and at least one electric dischargeamplifier device, connected between said circuits for the transmissionof signals from said input circuit to said output circuit, of meansprovided with an element movable between certain limits to vary thetransmission efliciency of said amplifier device, means for varying thesignal strength applied to said input circuit, said last named meansbeing operative to reduce the signal strength applied to the inputcircuit as the transmission efficiency of the amplifier device is beingreduced by said first named means, and a common means for operating saidsecond named means simultaneously with said first named means.

8. The combination with an electrical signal amplifier having an inputcircuit, an output circuit and at least one electric discharge amplifierdevice connected between said circuits for the transmission of signalsfrom said input to said output circuit, of means provided with anelement movable between said limits to vary the transmission eiiiciencyof said amplifier device, a variable resistor, said resistor having amovable element arranged to vary the signal strength applied to saidinput circuit, the movement 0I" said last named element being effectiveto vary the signal strength as the transmission efciency of theamplifier device is varied, and means connected with said movableelements for moving the same simultaneously.

9. The combination with an electrical signal amplifier having an inputcircuit, an output circuit and at least one electric discharge amplifierdevice connected between said circuits for the transmission of signalsfrom said input to said output circuits, of means having a commoncontrol and provided with movable elements for simultaneously varyingthe transmission efficiency of said amplifier device and the strength ofsignals received by said input circuit.

10. The combination with an electrical signal amplifier having an inputcircuit, an output circuit and at least one electric discharge amplifierdevice connected between said circuits for the fil) transmission ofsignals from said input to said output circuits, of simultaneouslyvariable means to vary the transmission emciency of said amplifierdevice and a signal voltage applied to said input circuit, the range ofmovement of the transmission eiiiciency varying means at least equallingthe range of control of the signal voltage varying means in a directionof movement to increase the transmission efficiency.

11. In the operation of radio receiving apparatus utilizing thermionictubes for amplifying radio frequency signal energy, the method ofeiiectingv control of Volume of reproduction, which comprises varyingthe transfer of radio frequency signal energy between predeterminedlimits, and simultaneously varying the amplification characteristic ofat least one of said tubes.

12. In the operation of radio receiving apparatus utilizing thermionictubes for amplifying radio frequency signal energy, the method ofreducing the volume of signals which comprises modifying theamplification characteristic of at least one of said tubes to decreasethe amplification of signal energy, and simultaneously decreasing thetransfer of signal energy.

13. In the operation of radio receiving apparatus utilizing thermionictubes for amplifying yradio frequency signal energy, the method ofreducing amplitude of signals which comprises varying the potentialapplied to a control electrode of at least one of said tubes andsimultaneously decreasing signal energy transferred to the input of saidone tube.

14. In the operation of radio receiving apparatus utilizing thermionictubes of the screen grid type for amplifying radio frequency signalenergy, the method o-f varying volume of signals which comprises varyingthe positive potential applied to the screen grid of at least one ofsaid tubes, and varying the signal energy transferred to the input ofsaid one tube.

15. Radio receiving apparatus comprising at least one thermionicamplifier tube of the screen grid type, radio frequency input and outputcircuits therefor, and a.- volume control element adjustable to varyboth the positive biasing potential of the screen grid of said tube andthe magnitude of transfer of signal energy to the input circuit thereof.

16. Radio receiving apparatus comprising a screen grid amplifier tube,input and output circuits for said tube, a resistance in the inputcircuit, a resistance in the screen grid circuit of said tube, means foreffecting iiow of direct current through said second resistance tomaintain the screen positively biased with respect to the cathode of thetube and a volume control element adjustable simultaneously to vary inthe same senses the effective magnitudes of said resistancescumulatively to effect change in Volume reproduction.

17. The method of controlling the output energy of a vacuum tubeamplifier which comprises simultaneously controlling amplification ofthe tube and the amount of input signal supplied thereto.

18. The method of controlling the output energy of a vacuum tubeamplifier which comprises simultaneously varying the bias potentialapplied to the input circuit of the tube and the amount of input signalsupplied thereto.

19. In an amplifier system, in combination, a vacuum -tube amplifier,resistance means for controlling the bias. voltage applied to the signalinput grid of said tube, resistance means for controlling the signalinput voltage applied thereto, and a single means for adjusting bothsaid control resistance means.

20. The method of controlling the output energy of an amplifier deviceincorporating at least one electronic tube, which method consists inreducing the amplifying action of the electronic tube and at the sametime reducing the amount of input signal energy applied to theamplifier.

21. The steps in the method of controlling the output energy ofamplifying apparatus which is coupled to a source of signal voltage andwhich includes at least one amplifying tube which comprise increasingthe amplifying action of the tube and simultaneously increasing thecoupling between the amplifier device and the source of signal voltagein order to increase the output energy of the amplier device anddecreasing the amplifying action of the tube and simultaneouslydecreasing the coupling between the ampliiier device and the source ofsignal energy to decrease the output energy of the amplifier device.

STUART BALLANTINE.

